I have a Xonar Essence ST working perfectly under Gentoo.
Recently I bought an H6 daughterboard (for analog surround output) and the trouble began.

Even though alsamixer recognizes the combination correctly as Xonar ST+H6,
the (master) playback volume control is gone with the H6 connected.
According to the loudness, the volumes seems to be at maximum level and I can't change it.

While this isn't that bad for the surround output (I can adjust the volume at the external amplifier),
I can't use my headphones anymore.
So I need something like a master volume control in alsamixer and wonder why it isn't there.

I have a recent kernel (3.7.2), the sound driver (snd-virtuoso) built as module and alsa (lib, headers, utils) 1.0.25 / 1.0.26 installed.